Backback and small day pack?

I would think so, unless you plan to carry everything you need in pockets, which may not be advisable. At minimum, you'll need money, ID, phone and/or camera, and perhaps water. I like having a bag to put these things in, plus room to put a sweater and small souvenirs. You may already have something you can use as a day bag. Just keep an eye on your things and don't leave it anywhere.

I usually use a crossover day bag from REI for two reasons — it’s handy to stash an umbrella, sunglasses, maps, etc and it’s also a handy place to put things I may buy during the day. If I don’t plan to shop and don’t have anything extra to take with me i don’t use a day bag, but, rather, carry a very small Baggali I purse that fits only my wallet, passport, lip gloss and phone.

For years I used the Rick Steves Civita backpack and enjoyed it, but the last few years I've grown tired of hauling a backpack everyday. I've switched to a Pacsafe Vibe 325 which is a sling bag that is low profile and easy to carry and the zippers lock to keep out the pick pockets. I can carry an umbrella, rain shell, collapsable bottle for water and guide book with realtive ease. Pacsafe also makes an even smaller one, Vibe 150 if you want to go minimalist.

Another one who uses a Pacsafe. This one:

https://www.pacsafe.com/venturesafe-150-gii-anti-theft-cross-body-pack/60160100.html?cgid=slingpacks#start=1

In it I can fit sunglass case, pouch containing CC's and cash that clips into an internal zippered pocket, my camera (which is about palm size) and/or phone, and Ive even managed to stuff a lightweight rain jacket in too!
However, I also carry this inside the pacsafze:

https://www.rei.com/product/136763/sea-to-summit-ultra-sil-travel-day-pack?sku=1367630002&store=30&cm_mmc=PLA_Google_LIA%7C404_183061%7C1367630002%7Cnone%7C9b2c4aef-0866-4f7d-b523-3a258c7c0c58%7Cpla-449912391812&lsft=cm_mmc:PLA_Google_LIA%7C404_183061%7C1367630002%7Cnone%7C9b2c4aef-0866-4f7d-b523-3a258c7c0c58&kclid=9b2c4aef-0866-4f7d-b523-3a258c7c0c58&gclid=EAIaIQobChMIuOPSwMWk3wIVBDBpCh3-3AoeEAQYASABEgIQuPD_BwE

Whoa that was a long link - Notice the picture of the backpack in its stuff sack. It weighs 2.5 oz and can hold 20L of gear. So if I stumble on something I want to bring back (groceries/picnic, layers of clothing I started the day with, etc) I pull this out and put them in there. That way Im not carrying a backpack all day (which I opted not to do due to weight, heat, and that feeling being overloaded), but can carry whatever I like
That said, Im a lightweight traveler, so this works well for me. YMMV ;)

Take it. You can decide whether you need to walk around with it every day. But there's another reason to have one. On the airplane and on the bus, you'll want a bag to carry the stuff you need for a comfortable journey - reading material, tissue, cough drops, snacks, etc.

Note, that the little plastic shopping bags that we are drowning in here in the US, are not always free or available like they are here. So a day pack also acts as a shopping bag when your out. You'll likely buy stuff for drinking and eating to carry back to the room, and you'll also need to carry things like postcards, cuckoo clocks and lederhosen that you pick up on the way. (humor attempt)

I think a carryon sized travel pack is usually too big for carrying around as a daypack for sightseeing, but others may disagree. The trouble I have with the "two backpack" concept is how you lug them both around. I realize that a lot of people "wear" the smaller daypack in the front with the larger heavier pack on the back. I suppose that works, but I'd feel like the Michelin Man doing it.

One option would be to buy a small, collapsible daypack and store it in the larger pack en route. Eddie Bauer and High Sierra make reasonably priced small backpacks that fold up into their own pockets. They're perfect for day trips and are light enough to carry all day, including additional items you might pick up.

My travel strategy, particularly on long flights, is to carry a rolling bag for the overhead and a small to midsized backpack or daypack that fits over the handle of the roller. I pack whatever I'll need on the flight in the smaller bag and store it under the seat. This eliminates having to get up and access the overhead every time I want something from my bag. It works really well for me as long as I pack light and strategically.

Another, pricier option would be one of the Osprey backpacks with zip off daypacks. I tried one of these a few years ago, but ended up selling it as the combined bulk and weight was uncomfortable. I felt like I was invading North Africa carrying the thing, but it probably works well for bigger guys. If you have an REI store locally, you can "try one on" there. Eagle Creek's Switchback uses the same concept, albeit on a small rolling bag. And relatively expensive, in my opinion.

If you pack smart, the collapsible daypack idea might be the way to go. It doesn't add a lot of weight or bulk and is perfect for carrying around all day.

Try to think of what you want to carry while out and about. Will these things be comfortable in your pockets? Do you want to carry a map or guide book? There are lots of options for you from a crossbody bag to a small backpack. Check out the red Oxx brand micro manager bag or one of their larger bags. If you normally carry things in your pockets - then just go with that. You can always buy some sort of bag while traveling - practical souvenir. I use a bag that doubles as a "flight" bag. A flight bag is a small, underseat bag where I keep things like tissue, small first aid kit, snacks, disposable plastic bags, etc. - so that I can easily access these things while flying.

Did I miss if this person is male or female? If female and you have a cross body purse, you may not need a day pack, but I usually bring one. I also bring a very lightweight little bag that can be used for groceries, picnics, whatever, that folds into itself and is smaller than a coin purse. I got mine at Eddie Bauer.

Female solo traveler and have been traveling with backpack and small day pack for years. Only modification has been my backpacks have evolved over time to be smaller than in previous years.
I want to be able to move quickly and freely under my own power in any situation. And of course never checking a bag on the way out, occasionally check my backpack on return if I have items not permitted in carry on.

I took with me the Civita Day bag and used it when traveling between cities and on all day hikes in Switzerland. Otherwise, I left it in my hotel room because I found it to be more of a burden than a help. My phone was in a front pants pocket and a small bi-fold wallet in another front pocket. I either have my camera (small point and shoot) in my hand or clip the camera in its case to a front belt loop. If you travel during the day with a lot of stuff, I would take it. IMO, "travel light" applies to day to day outings as well.

As a glasses-dependent person who is religious about wearing sunglasses anytime I'm outdoors, no matter what the weather, the only option for me is a cross-body bag, because I switch glasses far too often to fool around with a backpack. I would buy the lightest-weight cross-body bag I could find that provided whatever organization I needed. I am not remotely interested in an overweight purse, so I try to find something like nylon.

I don't think anyone has mentioned that you may be carrying around a rain jacket. It's early May, north of the Alps, after all. I'd want some sort of container I could cram that jacket into when I didn't need to wear it.

My "carryon" bag is a RS Classic bag that I combine with a small bike messenger bag (Patagonia Half Mass Courier Bag). The RS Classic is a maximum legal carryon sized bag that I push (and sometimes exceed) the 8 kg weight limit that some aircarriers specify. My Half Mass bike bag serves as my "personal" bag and holds my small computer, travel documents and essential stuff that I would not want gate checked. (Even if your carryon is size and weight compliant, if the overheads are full your carryon could get a trip in the cargo hold.)

I can wear the Half Mass bike bag in front and still carry the Classic as a back pack. I use the Half Mass bike bag as an walk about bag and if I want to splurge on baggage, I have a lightweight nylon grocery bag for daily purchases.

TY All for the great feedback. Lot's to consider. Oh and I am a 60 yr. old solo traveller...Someone asked the male/female part at least.

TY All for the great feedback. Lot's to consider. Oh and I am a 60 yr. old solo traveller...Someone asked the male/female part at least.

As you've decided to use a Backpack, I'd highly recommend doing some research on these and perhaps consider one that has a waist belt and allows for torso range adjustment. The waist belt is very important for balancing the load between the hips and the shoulders. Some of the brands you could look at are Eagle Creek or Osprey. If you have an REI or similar store in your area, talk to the sales staff there, as they can measure your torso length and suggest a good pack. Some travel Backpacks come with a detachable Daypack, and I find that's a good combination.